Sensing Circuit with Reduced Bias Clamp

ABSTRACT

A sensing circuit having a reduced bias clamp and method of operating the sensing circuit are provided. The sensing circuit may include a reference path and a sensing path. The sensing path may include a first transistor, clamping capacitor and a pair of switches. The reference path may include a second transistor, clamping capacitor and another pair of switches. A common gain stage receiving a bias voltage charges the clamping capacitors for the respective paths in a charging mode. The clamping capacitors may be charged in a serial or partially parallel manner during the charging mode. Each path may be coupled to a comparator, which may sense current or voltage changes between the paths during a sense mode. The sensing circuit may be configured to provide for sensing current or voltage changes between multiple sensing and/or reference paths in a parallel or serial manner.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application Ser. No. 61/787,614, entitled “Sensing Circuit with Reduced Bias Clamp,” filed on Mar. 15, 2013, which application is hereby incorporated by reference herein in its entirety.

BACKGROUND

Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones, and others. Semiconductor devices may include various types of components including transistors, integrated circuits, passive components and interconnects. The semiconductor industry continues to decrease the size of the components within semiconductor devices to improve power consumption, efficiency and speed of such devices. As semiconductor devices decrease in size, the components within the devices change to minimize the consumption of silicon area within the devices and improve performance.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a sensing circuit in accordance with an embodiment;

FIG. 2 illustrates a method for operating the sensing circuit of FIG. 1 in accordance with an embodiment;

FIG. 3 illustrates another method for operating the sensing circuit of FIG. 1 accordance with an embodiment; and

FIG. 4 illustrates another sensing circuit in accordance with another embodiment.

DETAILED DESCRIPTION

The making and using of the embodiments of the present disclosure are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosed subject matter, and do not limit the scope of the different embodiments. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

Embodiments discussed herein may provide a sensing circuit having a reduced bias offset or mismatch across sensing inputs for a comparator. The sensing inputs may share a gain device, which may be used to provide a clamping voltage for voltage controlled transistors on each of the sensing inputs. The shared gain device may reduce bias offset or mismatch between the sensing inputs for the comparator. Embodiments discussed herein may provide a sensing circuit that may be configured to provide sensing for read cells of various resistance-type memories such as, for example, magnetoresistive random access memory (“MRAM”), resistive RAM (“RRAM”) and/or phase-change RAM (“PCRAM”). Manufacturing yield and power consumption may be improved by providing a sensing circuit in accordance with embodiments discussed herein.

FIG. 1 illustrates a sensing circuit 100 in accordance with various embodiments. The sensing circuit 100 may include a sensing path and a reference path. The sensing path may provide a first sensing input S_(IN1) to a comparator 120 and the reference path may provide a second sensing input S_(INREF) to the comparator 120. The sensing path and the reference path may share a gain stage 110, which may receive a bias voltage V_(BIAS). In some embodiments, the sensing path may sense changes for a bit line of a memory cell and a reference bit line may be provided to the reference path.

In some embodiments, the sensing path includes a first pair of switches SW1, SW2 (a first switch SW1 and a second switch SW2), a first clamping capacitor C₁ and a first transistor T₁. The sensing path may receive a first input voltage V₁ and may provide the first sensing input S_(IN1) to the comparator 120. A first clamping voltage V_(C1) may be applied to the first clamping capacitor C₁ from the gain stage 110. The first capacitor C₁ may have a first terminal coupled to a control terminal of the first transistor T₁ and a second terminal coupled to ground GND. The control terminal may be a gate terminal of the first transistor T₁. The first terminal of the first capacitor C₁ may further be coupled to a first terminal of the second switch SW2. A second terminal of the second switch SW2 may be coupled to an output of the gain stage 110. The first transistor T₁ may have a first terminal coupled to a first terminal of the first switch SW1, which may also receive the first input voltage V₁. A second terminal of the first switch SW1 may be coupled to a first input of the gain stage 110. The first transistor T₁ may have a second terminal coupled to the first sensing input S_(IN1) of the comparator 120.

In some embodiments, the reference path includes a second pair of switches SW3, SW4 (a third switch SW3 and a fourth switch SW4), a second clamping capacitor C₂ and a second transistor T₂. The reference path may receive a reference input voltage V_(REF) and may provide the second sensing input S_(INREF) to the comparator 120. A second clamping voltage V_(C2) may be applied to the second clamping capacitor C₂ from the gain stage 110. The second capacitor C₂ may have a first terminal coupled to a control terminal of the second transistor T₂ and a second terminal coupled to ground GND. The control terminal may be a gate terminal of the second transistor T₂. The first terminal of the second capacitor C₂ may further be coupled to a first terminal of the fourth switch SW4. A second terminal of the fourth switch SW4 may be coupled to the output of the gain stage 110. The second transistor T₂ may have a first terminal coupled to a first terminal of the third switch SW3, which may also receive the reference input voltage V_(REF). A second terminal of the third switch SW3 may be coupled to the first input of the gain stage 110. The second transistor T₂ may have a second terminal coupled to the second sensing input S_(INREF) of the comparator 120.

The sensing circuit 100 may operate in a charge mode and a sense mode. During the charge mode, the gain stage 110 may be used to charge the respective first and second clamping capacitors C₁, C₂ to the corresponding first and second clamping voltages V_(C1), V_(C2). The bias voltage V_(BIAS) may be used by the gain stage 110 to provide a common clamping voltage for the sensing path and the reference at the start of the sense mode.

During the sense mode, the first transistor T₁ may be held on by the first clamping voltage V_(C1) as applied by the first clamping capacitor C₁. The second transistor T₂ may be held on by the second clamping voltage V_(C2) as applied by the clamping capacitor C₂. As the first input voltage V₁ may change for the sensing path, a first current I₁ through the sensing path may change, which may be registered at the first sensing input S_(IN1) for the comparator 120. The reference voltage V_(REF) may produce a reference current I_(REF) which may be registered at the second input S_(INREF) for the comparator 120. Changes in current between the sensing path and the reference path may be registered by the comparator 120, which may output the difference between the first current I₁ and the reference current I_(REF) as represented by the output signal AI. It is understood that the comparator 120 may also be used to measure voltage differences for the first and second path rather than current differences.

The charge mode may be operated in a serial charge mode or a partially parallel charge mode. In the serial charge mode, one of the paths, say, for example, the sensing path may be charged by closing the first pair of switches SW1 and SW2 and charging the first clamping capacitor C₁ to the first clamping voltage V_(C1). The gain stage 110 and the feedback loop created by the closed first pair of switches SW1 and SW2 may clamp the first input voltage V₁ to V_(BIAS), which may provide the first clamping voltage V_(C1) to the first clamping capacitor C₁. After the first clamping capacitor C₁ may be charged, the first pair of switches SW1 and SW2 may be opened and the second pair of switches SW3 and SW4 may be closed to charge the second clamping capacitor C₂ to the second clamping voltage V_(C2). The gain stage 110 and the feedback loop created by the second pair of switches SW3 and SW4 may clamp the reference voltage V_(REF) to V_(BIAS), which may provide the second clamping voltage V_(C2) to the second clamping capacitor C₂.

In the partially parallel charge mode, gain side switches for both the sense path and the reference path may be closed in order to charge the clamping capacitors together. One of the switches on the input side of the gain stage 110 may be closed for a path to fully charge the clamping capacitor for that corresponding path to the clamping voltage. After the clamping capacitor is charged, the both switches for the charged path may be opened and the remaining switch on the input side of the gain stage 110 for the uncharged path may be closed to charge the remaining clamping capacitor to its clamping voltage. When the remaining clamping capacitor is charged, the switches for that path may be opened and the sensing circuit 100 may be operated in the sense mode.

An example may begin as follows, the second switch SW2 and the fourth switch SW4 may be closed. The third switch SW3 may be closed to charge the second clamping capacitor C₂ to the second clamping voltage V_(C2). The gain stage 110 and the feedback loop created by the third and fourth switches SW3 and SW4 may clamp the reference voltage V_(REF) to V_(BIAS), which may provide the second clamping voltage V_(c2) to the second clamping capacitor C₂. Then, the third and fourth switches SW3 and SW4 may be opened and the first switch SW1 may be closed to charge the first clamping capacitor to the first clamping voltage V_(C1). The gain stage 110 and the feedback loop created by the first and second switches SW1 and SW2 may clamp the first input voltage V₁ to V_(BIAS), which may provide the first clamping voltage V_(C1) to the second first clamping capacitor C₁. When the first clamping capacitor C₁ is charged, the first and second switches SW1 and SW2 may be opened and the sensing circuit 100 may be operated in the sense mode.

The partially parallel charge mode may decrease the total charge time for charging the first and second clamping capacitors C₁, C₂ as compared to the serial charge mode, wherein each of the first and the second clamping capacitors C₁, C₂ are both charged from a lower charge potential in a serial manner. The tradeoff for the partially parallel charge mode, however, may be an increase in control circuitry to control the handshaking for the various switch configurations, as opposed to turning on and off a pair of switches for each path independently.

It is understood that either the sense path or the reference path may be charged in any order other than the orders described in the above examples without changing the scope of the embodiments of the sensing circuit 100 described herein.

In various embodiments, the first and second transistors may be NMOS and/or PMOS transistors, N-type or P-type bipolar junction transistors (“BJTs”), N-type or P-type field effect transistors (“FETs”), fin-type FETs, combinations thereof or the like. In various embodiments, any of the first, second, third, and/or fourth switches SW1-SW4 may be configured as transistor switches. In various embodiments, the gain stage 110 may be configured as a level shifter, an op-amp, an inverter, one or more NAND gates, one or more AND gates, one or more NOR gates, one or more OR gate, combinations thereof or the like. In an embodiment, the first and/or second clamping capacitors C₁, C₂ may be discrete devices or may be realized by changing the parasitic capacitance of the corresponding first and second transistors T₁, T₂.

FIG. 2 illustrates a method 200 for operating the sensing circuit 100 of FIG. 1 in accordance with various embodiments. As illustrated in FIG. 2, the method 200 may charge a first capacitor to a first charge voltage and set a first transistor to an on-state (block 202). The method 200 may set the first charge voltage using a first gain device. The method 200 may remove the first charge voltage from the first capacitor when the first capacitor is charged to the first charge voltage (block 204). The method 200 may charge a second capacitor to a second charge voltage and set a second transistor to an on-state (block 206). The method 200 may generate the second charge voltage using the first gain device. The method 200 may remove the second charge voltage from the second capacitor when the second capacitor is charged to the second charge voltage (208).

In some embodiments, the method 200 compares a first current from a first transistor and a second current from a second transistor to determine a difference between the first and second current. In some embodiments, the method 200 sets the first charge voltage using the first gain device, which may receive a first input voltage and a bias voltage. In some embodiments, the method 200 sets the second charge voltage using the first gain device, which may receive a second input voltage and the bias voltage. In some embodiments, the method 200 sets a first pair of switches to an on-state to charge the first capacitor to the first voltage. In other embodiments, the method 200 may set a second pair of switches to an on-state to charge the second capacitor to the second voltage.

FIG. 3 illustrates another method 300 for operating the sensing circuit 100 of FIG. 1. As illustrated in FIG. 3, the method may charge a first capacitor and a second capacitor to a first charge voltage and set a first transistor to an on-state (block 302). The method 300 may set the first charge voltage using a first gain device. The method 300 may remove the first charge voltage from the first capacitor and the second capacitor when the first capacitor is charged to the first charge voltage (block 304). The method 300 may charge the second capacitor to a second charge voltage and set a second transistor to an on-state (block 306). The method 300 may set the second charge voltage using the first gain device. The method 300 may remove the second charge voltage from the second capacitor when the second capacitor is charged to the second charge voltage (block 308).

In an embodiment, the method 300 compares a first current from the first transistor and a second current from the second transistor to determine a difference between the first current and the second current. In an embodiment, the method 300 sets the first charge voltage using the first gain device, which receives a first input voltage and a bias voltage. In an embodiment, the method 300 sets the second charge voltage using the first gain device, which receives a second input voltage and the bias voltage.

In various embodiments, a sensing circuit may be configured to provide reduced bias offset or mismatch for multiple pairs of sensing lines, otherwise referred to herein as multi-path sensing. The multiple pairs of sensing lines may provide for sensing changes between one or more input voltages and one or more reference voltages. FIG. 4 illustrates another sensing circuit 400 in accordance with various multi-path sensing embodiments. The sensing circuit 400, as shown in FIG. 4, is configured to provide sensing between a sensing input voltage V_(SENS1) and a plurality of reference input reference voltages V_(REF1)-V_(REFN). It is understood, however, that the sensing circuit 400 in accordance with the present embodiments may be configured to provide sensing between one or more input voltages (not shown) and one or more reference voltages.

As shown in FIG. 4, the sensing circuit 400 includes a sensing path and a plurality of reference paths 1-N coupled to a comparator 420. The sensing path and each of the plurality of reference paths 1-N share a common gain device 410, which receives a bias voltage V_(BIAS). The sensing path may provide a first sensing input IN_(SENS1) to the comparator 420. The sensing path may include a pair of sensing path switches SW_(SENS1), SW_(SENS2) (a first switch SW_(SENS1) and a second switch SW_(SENS2))_(,) a sensing path clamping capacitor C_(SENS1), and a sensing path transistor T_(SENS1). The configuration of the components for the sensing path of FIG. 4 is similar to that as described for the sensing path of the sensing circuit 100 as shown in FIG. 1. Thus, a detailed description of the connections for the sensing path components (SW_(SENS1), SW_(SENS2), T_(SENS1), C_(SENS1)) of the sensing circuit 400 will not be repeated for the sake of brevity.

Each of the plurality of reference paths 1-N for the sensing circuit 400 receives a corresponding reference input voltage V_(REF1)-V_(REFN). A first reference path will be described in relation to the first reference input voltage V_(REF1). The first reference path provides a first reference sensing input IN_(SENSREF1) to the comparator 420 and includes a first pair of reference path switches SW_(REF1.1), SW_(REF1.2) (a first switch SW_(REF1.1), and a second switch SW_(REF1.2)), a first reference path clamping capacitor C_(REF1), and a first reference path transistor T_(REF1). The configuration of the components for the first reference path of FIG. 4 is similar to that as described for the reference path of the sensing circuit 100 as shown in FIG. 1. Thus, a detailed description of the connections for the first reference path components (SW_(REF1.1), SW_(REF1.2), T_(REF1), C_(Ref1)) of the sensing circuit 400 will not be provided for the sake of brevity.

Each of the remaining reference paths 2-N may be configured in a manner similar to that as described for the first reference path and may share connections to the gain device 410 to reduce bias offset or mismatch between the various sensing paths for the comparator 420. For example, a second reference path may include a second pair of reference path switches SW_(REF2.1), SW_(REF2.2), a second reference path clamping capacitor C_(REF2) and a second reference path transistor T_(REF2). The second reference path receives a second reference voltage V_(REF2) and provides a second reference sensing input IN_(SENSREF2) to the comparator 420.

As shown in FIG. 4, the switch SW_(REF2.1) has one terminal that shares a connection to an output of the gain device 410 and another terminal that is coupled to a first terminal of the second reference path clamping capacitor C_(REF2) and is coupled to a gate terminal of the second reference path transistor T_(REF2). The second reference path transistor T_(REF2) has a first terminal coupled to the second reference sensing input IN_(SENSREF2) to the comparator 420. The second reference path transistor T_(REF2) has a second terminal coupled to an input of the gain device 410 through the switch SW_(REF2.2).

Each of the remaining plurality of N reference paths may be configured in a manner similar to that described for the first and second reference paths wherein each of the remaining N reference paths may share similar connections to the gain device 410. In this manner, the configuration of the sensing circuit 400 may reduce bias offset or mismatch between the sensing path and each of the reference paths 1-N.

Similar to the sensing circuit 100 of FIG. 1, the sensing circuit 400 of FIG. 4 may operate a charge mode and a sense mode. During the charge mode, the gain device 410 charges the clamping capacitor C_(SENS1) for the sensing path to a clamping voltage V_(C1) and charges the clamping capacitors C_(REF1)-C_(REFN) for each of the corresponding 1-N reference paths to V_(CREF1)-V_(CREFN), respectively. The bias voltage V_(BIAS) is used by the gain stage 410 to provide an approximately common clamping voltage for the sensing path and each of the corresponding 1-N reference paths at the start of a sense mode between the sense path and a corresponding reference path.

During a sense mode between the sensing path and a corresponding reference path, the sensing circuit 400 operates to detect changes between the sensing path and the corresponding reference path. The changes may be detected as current changes or voltage changes by the comparator 420. For example, the comparator 420 may receive and compare a current I_(SENS1) at its input IN_(SENS1) and a current I_(REF1) at its input IN_(SENSREF1). The comparator 420 outputs a difference between the currents as represented by the output signal ΔI. A control circuit (not shown) may be configured to cycle the sensing circuit 400 through the corresponding 1-N reference paths in order to compare corresponding differences (current or voltage) between the sensing input and a corresponding reference path.

In an embodiment, the sensing circuit 400 may be configured to provide for sensing between multiple sensing paths (not shown) and multiple reference paths. In an embodiment, a control circuit (not shown) may be configured to cycle or step the sensing circuit 400 through corresponding pairs of sensing path and reference path comparisons. The cycling or stepping may be performed serially or non-serially between the various comparisons. In this manner, the sensing circuit 400 may provide for sensing changes between one or more bit lines of a memory cell and one or more reference bit lines.

In another embodiment, the comparator 420 may be configured with a plurality of N current mirrors (not shown) to replicate the current I_(SENS1) for the sensing path across the comparator 420 in order to perform parallel multipath comparisons between the sensing path current I_(SENS1) and each of the N reference path currents I_(REF1)-I_(REFN) in parallel. In such an embodiment, the comparator 420 may further be configured to provide N output difference currents ΔI corresponding to each of the multi-path comparisons. Parallel multipath sensing may reduce the total access time for performing multiple comparisons between multiple paths as opposed to cycling through multiple comparisons serially. The parallel multipath sensing may increase the complexity of the comparator configuration, but the time savings may outweigh the complexity costs for timing critical applications.

For the charge mode, the sensing path and each of the corresponding reference paths 1-N for the sensing circuit 400 may be charged in a serial or in a partially parallel manner similar to the charging as described for the sensing circuit 100 of FIG. 1. For example, in a serial charging mode, the clamping capacitor C_(SENS1) for the sensing path is charged to the clamping voltage V_(C1) by closing the sensing path switches SW_(SENS1), SW_(SENS2). Following the sensing path charging, for example, the first reference path clamping capacitor C_(REF1) is charged to the clamping voltage V_(CREF1) by closing the switches SW_(REF1.1), SW_(REF1.2) for the first reference path.

In another example, for partially parallel charging, the clamping capacitor C_(SENS1) for the sensing path may be charged to the clamping voltage V_(C1) by closing the sensing path switches SW_(SENS1), SW_(SENS2). The reference path switch SW_(REF1.1) coupled to the output of the gain device 410 for the first reference path may be closed to provide a partial charge to the first reference path clamping capacitor C_(REF1). After the clamping capacitor C_(SENS1) is charged to the clamping voltage V_(C1), the sensing path switches SW_(SENS1), SW_(SENS2) are opened and the reference path switch SW_(REF1.2) closes to fully charge the first reference path capacitor C_(REF1) to the clamping voltage VC_(REF1). Any of the paths of the sensing circuit 400 may be charged using serial or partially parallel charge modes. For multipath sensing, the various clamping capacitors for the paths may all be charged and then sensing may be performed in a parallel or stepped manner as described above.

In various embodiments, the transistors for the sensing circuit 400 may be NMOS and/or PMOS transistors, N-type or P-type bipolar junction transistors (“BJTs”), N-type or P-type field effect transistors (“FETs”), fin-type FETs, combinations thereof or the like. In various embodiments, the switches for the sensing circuit 400 may be configured as transistor switches. In various embodiments, the gain stage 410 may be configured as a level shifter, an op-amp, an inverter, one or more NAND gates, one or more AND gates, one or more NOR gates, one or more OR gate, combinations thereof or the like. In an embodiment, the one or more of the clamping capacitors for the sensing circuit 400 may be discrete devices or may be realized by changing the parasitic capacitance of the transistor for a corresponding path.

In an embodiment, a circuit is provided. The circuit may include a gain device having a first input and a first output; a comparator having a first input, a second input and a first output; a first transistor and a second transistor. The first transistor may have a first control terminal. The first control terminal of the first transistor may be coupled to a first terminal of a first capacitor and a first terminal of a first switch. The first switch may have a second terminal coupled to the first output of the gain device. The first transistor may have a second terminal coupled to the first input of the comparator and a third terminal coupled to a first terminal of a second switch. The second switch may have a second terminal coupled to the first input of the gain device. The third terminal of the first transistor may be configured to receive a first voltage. The second transistor may have a first control terminal. The first control terminal of the second transistor may be coupled to a second capacitor and to a first terminal of a third switch. The third switch may have a second terminal coupled to the first output of the gain device. The second transistor may have a second terminal coupled to the second input of the comparator. The second transistor may have a third terminal coupled to a first terminal of a fourth switch. The fourth switch may have a second terminal coupled to the first input of the gain device. The third terminal of the first transistor may be configured to receive a second voltage.

In an embodiment, a method is provided. The method may include charging a first capacitor to a first charge voltage and setting a first transistor to an on-state, wherein the first charge voltage is generated by a first gain device; removing the first charge voltage from the first capacitor when the first capacitor is charged to the first charge voltage; charging a second capacitor to a second charge voltage and setting a second transistor to an on-state, wherein the second charge voltage is generated by the first gain device; and removing the second charge voltage from the second capacitor when the second capacitor is charged to the second charge voltage.

In an embodiment, another method is provided. The method may include charging a first capacitor and a second capacitor to a first charge voltage and setting a first transistor to an on-state, wherein the first charge voltage is generated by a first gain device; removing the first charge voltage from the first capacitor and the second capacitor when the first capacitor is charged to the first charge voltage; charging the second capacitor to a second charge voltage and setting a second transistor to an on-state, wherein the second charge voltage is generated by the first gain device; and removing the second charge voltage from the second capacitor when the second capacitor is charged to the second charge voltage.

Although the present embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, it will be readily understood by those skilled in the art that the structures and ordering of steps as described above may be varied while remaining within the scope of the present disclosure.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A circuit comprising: a gain device having a first input and a first output; a comparator having a first input, a second input and a first output; a first transistor having a first control terminal, the first control terminal of the first transistor coupled to a first terminal of a first capacitor and a first terminal of a first switch, the first switch having a second terminal coupled to the first output of the gain device, the first transistor having a second terminal coupled to the first input of the comparator, the first transistor having a third terminal coupled to a first terminal of a second switch, the second switch having a second terminal coupled to the first input of the gain device; and a second transistor having a first control terminal, the first control terminal of the second transistor coupled to a second capacitor and to a first terminal of a third switch, the third switch having a second terminal coupled to the first output of the gain device, the second transistor having a second terminal coupled to the second input of the comparator, the second transistor having a third terminal coupled to a first terminal of a fourth switch, the fourth switch having a second terminal coupled to the first input of the gain device.
 2. The circuit of claim 1, wherein the gain device is at least one of an operational amplifier, a level shifter or an inverter.
 3. The circuit of claim 1, wherein the first and the second transistor both N-type or P-type transistors.
 4. A method comprising: charging a first capacitor to a first charge voltage and setting a first transistor to an on-state, wherein the first charge voltage is generated by a first gain device; removing the first charge voltage from the first capacitor when the first capacitor is charged to the first charge voltage; charging a second capacitor to a second charge voltage and setting a second transistor to an on-state, wherein the second charge voltage is generated by the first gain device; and removing the second charge voltage from the second capacitor when the second capacitor is charged to the second charge voltage.
 5. The method of claim 4, further comprising comparing a first current from the first transistor to a second current from the second transistor to determine a difference between the first current and the second current.
 6. The method of claim 4, wherein the first gain device receives a first input voltage and a bias voltage to generate the first charge voltage.
 7. The method of claim 4, wherein the first gain device receives a second input voltage and a bias voltage to generate the second charge voltage.
 8. The method of claim 4, wherein the first gain device is at least one of an operational amplifier, a level shifter or an inverter.
 9. The method of claim 4, the charging the first capacitor further comprising: setting a first pair of switches to an on-state to form a first feedback loop with the first transistor and the first gain device to charge the first capacitor to the first voltage.
 10. The method of claim 4, the charging the second capacitor further comprising: setting a second pair of switches to an on-state to form a second feedback loop with the second transistor and the first gain device to charge the second capacitor to the second voltage.
 11. The method of claim 4, wherein at least one of the first capacitor and the second capacitor are a discrete capacitor.
 12. The method of claim 11, wherein the first capacitor is a parasitic capacitance of the first transistor.
 13. The method of claim 11, wherein the second capacitor is a parasitic capacitance of the second transistor.
 14. A method comprising: charging a first capacitor and a second capacitor to a first charge voltage and setting a first transistor to an on-state, wherein the first charge voltage is generated by a first gain device; removing the first charge voltage from the first capacitor and the second capacitor when the first capacitor is charged to the first charge voltage; charging the second capacitor to a second charge voltage and setting a second transistor to an on-state, wherein the second charge voltage is generated by the first gain device; and removing the second charge voltage from the second capacitor when the second capacitor is charged to the second charge voltage.
 15. The method of claim 14, further comprising comparing a first current from the first transistor to a second current from the second from the second transistor to determine a difference between the first current and the second current.
 16. The method of claim 14, wherein the first gain device receives a first input voltage and a bias voltage to generate the first charge voltage.
 17. The method of claim 14, wherein the first gain device receives a second input voltage and a bias voltage to generate the second charge voltage.
 18. The method of claim 14, wherein the charging the first capacitor and the second capacitor to the first voltage further comprising: setting a first pair of switches to an on-state to form a first feedback path with the first transistor and the first gain device to charge the first capacitor to the first voltage; and setting a first switch of a second pair of switches to an on-state to charge the second capacitor to the first voltage.
 19. The method of claim 18, wherein the charging the second capacitor to the second voltage further comprising: holding the first switch of the second pair of switches in the on-state; and setting a second switch of the second pair of switches to an on-state to charge to form a second feedback path with the second transistor and the first gain device to charge the second capacitor to the second voltage.
 20. The method of claim 14, wherein at least one of the first capacitor and the second capacitor are a discrete capacitor. 